For discussion: A slightly more flexible skip plugin

I'm submitting this pull request to generate discussion around the plugin mechanism and to have some example code that will ideally help drive out what you're building.  I would love whatever relevant feedback you can provide, and hopefully having actual code will help us talk more clearly.

Generally, though: I like where this is going. 

If you think this might be a feature that you'd like to merge into the paella core, great! I'll make whatever changes you think are necessary to make that happen. Otherwise, I'm happy to continue managing this as a custom plugin.

I will comment in-line with questions about implementation details and design decisions.
We have discussed the possibility of use a list of plugins in the configuration file, and we like the idea. Maybe we need to think a little bit what occurs if a plugin is not listed in the plugin list (by default, all the plugins are loaded, it's not necessary to add it to the config.json file explicitly), but we going to work in this approach for the next Paella major release.